The present invention relates to the field of control engineering, in particular to controlling the operation of a wind turbine. Furthermore, the invention relates to wind turbines having a control system.
Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
A wind turbine can only extract a certain percentage of the power associated with the wind, up to the so-called maximum “Betz limit” of 59%. This fraction is described as the power coefficient. The value of the real power coefficient during operation is a function of the form, wind speed, rotation speed and pitch of the specific wind turbine. Assuming all other operational variables to be constant, this coefficient has only one maximum point at a fixed wind speed as the rotational speed is varied. It is therefore known to adjust the rotation speed of the turbine's rotor to this maximum value, that is called “optimal rotation speed” herein. From the rotation speed, the tip-speed ratio is directly derived:
The characteristics of the power coefficient are normally expressed in dependency of the tip-speed-ratio λ (or TSR), which is defined as:
  λ  =                    v        p            v        =                  Ω        ·        R            v      wherein νp is the tip-speed of the one or more turbine blades, R is the turbine rotor radius, Ω is the rotational turbine angular velocity and ν is the wind speed. The optimal rotation speed for maximum power output thus yields an optimal tip-speed ratio λmax or TSRmax.
Known wind turbines have a plurality of mechanical and electrical components. Each electrical and/or mechanical component may have independent or different operating limitations, such as current, voltage, power, and/or temperature limits, than other components. Moreover, known wind turbines typically are designed and/or assembled with predefined rated power limits. To operate within such rated power limits, the electrical and/or mechanical components may be operated with large margins for the operating limitations. Consequently, many turbines do not run at optimal tip-speed ratio λmax or TSRmax, but at a lower tip-speed ratio. This is, amongst other factors, due to the fact that individual component margins are typically reached for the standard operating conditions taken into account during the design phase of the turbine. If the operating conditions are different from the design conditions, the turbine may thus run with less power output than possible. Such operation may result in inefficient wind turbine operation, and a power generation capability of the wind turbine may be underutilized depending on operating conditions.